For the first time in 18 years, maintenance workers for Montreal's public transit system walked off the job Monday, poking significant holes in the city's bus and Metro network schedule and causing headaches for commuters.

More unions may be joining the movement. The one that represents bus and Metro car drivers for the Société de transport de Montréal (STM) has approved a strike mandate as well. 

The maintenance workers with the Syndicat de transport de Montréal are trying to renew their collect…